Same-aperture any-frequency simultaneous transmit and receive communication system

ABSTRACT

A same-aperture any-frequency simultaneously transmit and receive (STAR) system includes a signal connector having a first port electrically coupled to an antenna, a second port electrically coupled to a transmit signal path, and a third port electrically coupled to receive signal path. The signal connector passes a transmit signal in the transmit signal path to the antenna and a receive signal in the receive signal path. A signal isolator is positioned in the transmit signal path to remove a residual portion of the receive signal from transmit signal path. An output of the signal isolator provides a portion of the transmit signal with the residual portion of the receive signal removed. A signal differencing device having a first input electrically coupled to the output of the signal isolator and a second input electrically coupled to the third port of the signal connector subtracts a portion of the transmit signal in the receive signal path thereby providing a more accurate receive signal.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/901,729, entitled “Same-Aperture Any-Frequency Simultaneous Transmitand Receive Communication System”, filed Feb. 21, 2018, which is acontinuation of U.S. patent application Ser. No. 13/844,180, entitled“Same-Aperture Any-Frequency Simultaneous Transmit and ReceiveCommunication System”, filed on Mar. 15, 2013, now U.S. Pat. No.9,935,680, which claims priority to U.S. Provisional Patent ApplicationNo. 61/755,044, filed on Jan. 22, 2013, entitled “Single-Aperture, FullDuplex Communication System” and to U.S. Provisional Patent ApplicationNo. 61/677,366 filed on Jul. 30, 2012, entitled “Signal Canceller andSimultaneous Transmit and Receive System with Signal Processing.” Theentire contents of U.S. patent application Ser. No. 15/901,729, U.S.Pat. No. 9,935,680, and U.S. Provisional Patent Application Ser. Nos.61/755,044 and 61/677,366 are herein incorporated by reference.

The section headings used herein are for organizational purposes onlyand should not to be construed as limiting the subject matter describedin the present application in any way.

INTRODUCTION

It is generally assumed in communications that it is not possible tosimultaneously transmit and receive (STAR) in the same frequency band.Recently this basic tenet has begun to be challenged by several groupsthat have reported prototype STAR systems. Researchers at Purdue in, forexample, A. Wegener and W. Chappell, “Simultaneous transmit and receivewith a small planar array,” IEEE MTT-S Int. Microwave Symp. Dig.,Montreal, June 2012, and researchers at Stanford in, for example, J.Choi, et al., “Achieving Single Channel, Full Duplex WirelessCommunication,” Proc. Int. Conf. Mobile Computing and Networking, NewYork, 2010 have proposed arrangements of multiple antenna elements inwhich the receive antenna is located in a null of the transmit antennapattern to realize ˜40 dB of transmit-to-receive (T/R) isolation.

Signal processing was then used to extend the T/R isolation to ˜60-70dB. A group at Rice University using single, separate transmit andreceive antennas, computed the required cancelling signal and used it tocancel the transmit signal before it reached the analog-to-digitalconverter. See A. Sahai, B. Patel and A. Sabharwal, “Asynchronousfull-duplex wireless,” Proc. Int. Conf. on Communication Systems andNetworks, pp. 1-9, 2012. This group reported up to 79 dB suppression. Akey limitation of these approaches is the limited bandwidth over whichsufficient T/R isolation can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teaching, in accordance with preferred and exemplaryembodiments, together with further advantages thereof, is moreparticularly described in the following detailed description, taken inconjunction with the accompanying drawings. The skilled person in theart will understand that the drawings, described below, are forillustration purposes only. The drawings are not necessarily to scale,emphasis instead generally being placed upon illustrating principles ofthe teaching. The drawings are not intended to limit the scope of theApplicant's teaching in any way.

FIG. 1 illustrates a block diagram of a same-aperture any-frequencysimultaneously transmit and receive (STAR) system using knowntechnology.

FIG. 2 shows a block diagram of a same-aperture any-frequency STARsystem according to the present teaching.

FIG. 3A illustrates a signal connector in which the RF impedance ismatched at each of the three ports.

FIG. 3B illustrates a signal connector wherein the path to thedifferencing device presents a high RF impedance at the port labeledPort 3, which minimizes the signal loss in the connector between Port 1and Port 2.

FIG. 3C illustrates a signal connector wherein the output of thetransmit signal path presents a high RF impedance at the port labeledPort 1, which minimizes the signal loss in the connector between Port 2and Port 3.

FIG. 3D illustrates a signal connector including a fast switch.

FIGS. 4A and 4B illustrate an active electronic differencing device thattakes the difference of two voltages and two currents.

FIG. 4C illustrates a passive electronic differencing device.

FIG. 4D illustrates one embodiment of a photonic differencing devicethat includes a balanced-drive optical modulator which produces amodulated output that is proportional to the sum or difference betweenthe signals that are applied to the electrodes.

FIG. 5A illustrates an electronic voltage-source-based isolator that canbe used with the same-aperture any-frequency STAR system of the presentteaching.

FIG. 5B illustrates a current-source-based signal isolator used with thesame-aperture any-frequency STAR system of the present teaching.

FIG. 5C illustrates a passive electronic isolator including anon-reciprocal RF two-port device that can be used with thesame-aperture and frequency STAR system of the present teaching.

FIG. 5D illustrates a photonic isolator that can be used with thesame-aperture any-frequency STAR system according to the presentteaching.

FIGS. 6A-6D illustrate signal processors that can be used with thesame-aperture any-frequency STAR system according to the presentteaching.

FIG. 7A shows an adjustment circuit that adjusts the magnitude and phaseof the transmit signal that can be used with the same-apertureany-frequency STAR system according to the present teaching.

FIG. 7B shows an adjustment circuit that adjusts the in-phase andquadrature components of the transmit signal that can be used with thesame-aperture any-frequency STAR system according to the presentteaching.

FIG. 8 illustrates a block diagram of one exemplary embodiment of afront-end system that includes the matched impedance signal connector,the photonic differencing circuit and the electronicvoltage-source-based isolator described herein.

FIG. 9 illustrates a block diagram of one exemplary embodiment of afront-end system that includes the signal connector to which a highimpedance is presented by the output of the transmit signal path, thepassive electronic differencing device, and the current-source-basedisolator described herein.

FIG. 10 illustrates a block diagram of one exemplary embodiment of afront-end system that includes the connector with a high impedanceapplied to the output receive signal port by the ‘+’ port of the activeelectronic differencing device, and the voltage-source-based isolatordescribed herein.

FIG. 11 shows a same-aperture any-frequency STAR system using a fastswitch as a signal connector.

FIG. 12 shows a block diagram of a same-aperture any-frequency STARsystem 1200 using digital signal processing 1202 to augment the examplefront end system shown in FIG. 10.

FIG. 13 shows a block diagram of a same-aperture any-frequency STARsystem illustrating how analog signal processing could be used toaugment the example front end system shown in FIG. 10.

FIG. 14 illustrates a subset of hardware in the same-apertureany-frequency STAR system described in connection with FIG. 2 that isuseful for some embodiments when the transmit signal strength is only asstrong as or weaker than the receive signal.

FIG. 15 illustrates one exemplary embodiment of the system described inFIG. 14, including a signal connector to which a high impedance ispresented by the output of the photonic isolator in the transmit signalpath, and in which conventional digital signal processing is used toremove the transmit signal from the receive path after all signals arefrequency down-converted and then converted from the analog to digitaldomain.

FIG. 16 illustrates a system that generates a reference copy of aninterfering signal according to the present teaching.

FIG. 17 illustrates results of a simulation of the architecture in FIG.16, whereby the output of a 1-bit quantizer produces a copy of thehigh-power interferer at 100 MHz, allowing its subtraction from thelower-power 107-MHz SOI in a differencing device.

FIG. 18 is a plot that shows the relationship between thesignal-to-interferer ratio at the antenna to the number of bits ofquantization that we can use without having to worry about suppressingthe SOI.

FIG. 19 illustrates a block diagram of a system according to the presentteaching that uses a self-generated reference in an interferencecanceller.

FIG. 20 illustrates a system according to the present teaching formitigating the effect of signals being transmitted not only by theantenna element attached to the front-end shown by the collection ofhardware inside dashed box but also by the N−1 other radiating elementsin an array of N such radiating elements.

DESCRIPTION OF VARIOUS EMBODIMENTS

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the teaching. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

It should be understood that the individual steps of the methods of thepresent teachings may be performed in any order and/or simultaneously aslong as the teaching remains operable. Furthermore, it should beunderstood that the apparatus and methods of the present teachings caninclude any number or all of the described embodiments as long as theteaching remains operable.

The present teaching will now be described in more detail with referenceto exemplary embodiments thereof as shown in the accompanying drawings.While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives, modifications and equivalents, as willbe appreciated by those of skill in the art. Those of ordinary skill inthe art having access to the teaching herein will recognize additionalimplementations, modifications, and embodiments, as well as other fieldsof use, which are within the scope of the present disclosure asdescribed herein.

For decades, there existed only microwave circulators to simultaneouslyconnect the transmit and receive paths to a common antenna. Microwavecirculators are passive components with three ports arranged in awaveguide ring around a ferrite disk that induces a direction-dependentphase shift, causing the two counter-circulating halves of the wave toadd up constructively at the next port in one circumferential directionalong the ring but destructively at the next port in the otherdirection. A ferrite circulator is an inherently narrow-band devicebecause it depends on summing and differencing the RF phase of twowaves. Designers have found ways to widen a ferrite circulator'sbandwidth in exchange for some loss of its perfect unidirectionality atits center design frequency. Ferrite circulators are now commerciallyavailable from multiple vendors with ˜20 dB of port 1-3 isolation overan octave-wide band.

To enable single-aperture STAR applications, separate groups ofresearchers recently hit upon two active circulator designs. Anelectronic circulator has achieved up to 40 dB T/R isolation, albeitover only about 10% bandwidth at X-band. A description of the electroniccirculator's principle of operation is described in S. Cheung, et al.,“MMIC-based quadrature hybrid quasi-circulators for simultaneoustransmit and receive,” IEEE Trans. Microwave Theory Tech., vol. 58, pp.489-497, March 2010.

The second new type of device is based on photonics and hence it isreferred to herein as a photonic circulator. As described herein, thisnew photonic component performs two additional functions beyond those ofa conventional ferrite circulator. For this reason, we refer to the newphotonic component as a TIPRx, for Transmit-Isolating Photonic Receiver.

Several years ago Photonic Systems, Inc., the assignee of the presentapplication, began to investigate a more challenging yet potentiallymore widely applicable STAR configuration, which is STAR via the sameantenna element and in the same polarization.

It is well known in the communications art that to simultaneouslytransmit and receive via the same aperture, one must use either time,frequency, or code multiplexing. Time multiplexing involves inserting aswitch so that either the transmitter or the receiver is connected tothe antenna. Frequency multiplexing involves inserting a diplexer and/orfilters so that the transmit and the receive signals occupy disjointportions of the RF spectrum. Code multiplexing uses orthogonal codes forthe transmit and receive signals; the relatively limited degree oforthogonality that can be realized, however, often requires codemultiplexing to be augmented with frequency multiplexing to achievesufficient transmit-to-receive (T/R) isolation. Thus, persons skilled inthe art generally agree that it is not possible to simultaneouslytransmit and receive via the same aperture using the same portion of theRF spectrum at the same time.

FIG. 1 illustrates a block diagram of a same-aperture any-frequencysimultaneously transmit and receive (STAR) system 100 using knowntechnology. The isolation is provided by the ferrite circulator 102. Animpedance matching network 104 is connected to one port of thecirculator 102 that receives the reception signal. The transmit signalis applied to the second port of the circulator 102. A 2-way RF combiner106 is used to combine the receive signal that includes a portion of thetransmit signal with a leakage suppression signal.

A key parameter to achieving same-aperture any-frequency STAR is the T/Risolation; systems typically would require >60 dB of T/R isolation. Thesystem 100 of FIG. 1 shows the two main paths by which the strongtransmit signal can enter the receive path. One path is leakage throughthe circulator 102. Typical T/R isolation of a ferrite circulator is inthe range 15-20 dB. It is well known that one can improve the isolationof a circulator by constructing a second path and designing this secondpath so that the transmit signal in this path destructively interfereswith the circulator leakage. However, the bandwidth over which thisisolation improvement can be achieved is severely limited. The otherprimary path by which the transmit signal can enter the receive path isthrough reflection off the antenna impedance. The return loss ofstate-of-the-art antennas is also in the range of −15 to −20 dB. Oneapproach to improve the antenna return loss is to use an impedancematching circuit. It can be shown, however, that the required degree ofimprovement in impedance match is beyond that which is physicallyrealizable, which is set by the Bode-Fano limit. One aspect of thepresent teaching relates to methods and apparatus for improving the T/Risolation in same-aperture any-frequency STAR systems over asufficiently wide bandwidth for practical systems.

FIG. 2 shows a block diagram of a same-aperture any-frequency STARsystem 200 according to the present teaching. The system 200 includes athree-port signal connector 202 that passes both transmit and receivesignals. The signal connector 202 connects three signal paths, one fromand to the antenna 204, one from the output of transmit path 205 and oneto the input to receive path 206. In practical systems, the relativeimpedance seen by signals propagating in these paths is important. Asignal isolator 208 is present in the transmit signal path 205. A signaldifferencing device or equivalently a signal subtractor 210 connects thesignal isolator 208 and the signal connector 202. The system alsoincludes various optional feedback components to improve the T/Risolation.

One input of the differencing device 210 is connected to the receivepath 206. Another input of the differencing device 210 is connected tothe transmit signal path 205 that ideally has no residual receivesignal. The isolator 208 connected to the transmit signal path 205 isdesigned to isolate any residual receive signal so that a clean copy ofthe transmit signal is applied to the differencing device 210. Inoperation, the differencing device 210 subtracts out the large transmitsignal leaving just the receive signal.

If the transmit signal environment is sufficiently stable, it ispossible to provide a transmit signal of fixed complex value to thesecond port of the differencing device 210. However, in many practicalsame-aperture any-frequency STAR systems, the transmit environmentaround the antenna 204 will change as a function of time, which in turnwill cause the complex value of the transmit signal reflected by theantenna to change. In these situations it is desirable to include asignal processor 212 to determine the precise complex value of thetransmit signal that should be fed to the second terminal of thedifferencing device 210 so as to minimize the residual transmit signalthat is present in the receive path. A transmit signal adjustmentcircuit 214 is used to set the complex value of the transmit signal.

FIGS. 3A-3D illustrate four different signal connectors that can be usedwith same-aperture any-frequency STAR systems according to the presentteaching. Referring to FIG. 2 and FIGS. 3A-3D, the impedance at eachport of the signal connector can be designed to match the impedance ofthe component that is connected to that port. An impedance match at eachport can be achieved in numerous ways known in the art. For example,lumped element resistive dividers, traveling wave resistive (Wilkinson)dividers, and 180 degree hybrid couplers can be used to impedance matchthe port. FIG. 3A illustrates a signal connection 300 where all threeports of the signal connector 300 are impedance-matched to the paths towhich they are connected.

FIG. 3B illustrates a signal connector 320 that is presented with a highRF impedance at the input to the differencing device 210, and thereforeR_(diff)>R_(antenna) and R_(diff)>R_(isolator). Hence, the antenna 204impedance provides the primary load to the output of the transmit signalpath 205, which means more of the transmit power is delivered to theantenna 204 than is delivered to the receive path 206, which is highlydesirable for many applications.

FIG. 3C illustrates a signal connector 340 that is presented with a highRF impedance at the output of the transmit signal path 205, so thatR_(isolator)>R_(diff) and R_(isolator)>R_(antenna). In this signalconnector 340, the transmit power is divided between the antenna 204 andthe input to the differencing device 210 in proportion to the relativeimpedances of these two devices, represented by R_(antenna) andR_(diff), respectively. In the special sub-case whereR_(antenna)=R_(diff), the maximum receive power will be delivered to theinput of the differencing device 210, which is often desired to achievethe maximum receiver sensitivity.

FIG. 3D illustrates a signal connector 360 including a fast switch.Using the fast switch can eliminate several of the system components. Insome embodiments, the fast switch signal connector 360 eliminates theneed for the differencing device 210 and isolator 208. The use of thefast switch can also eliminate the need for the signal processor 212 andtransmit signal adjustment circuit 214.

FIGS. 4A-4D illustrate four different differencing devices 210 (FIG. 2).Referring to FIGS. 2-4, FIGS. 4A and 4B illustrate an active electronicdifferencing device 400, 420 that takes the difference of two voltagesand two currents. One example of such active differencing devices 400,420 is differential or balanced amplifiers. The active differencingdevices 400, 420 typically provide gain, which is well known to beadvantageous if it is desired to achieve a low noise figure for thereceive signal. The active differencing devices 400, 420 can be realizedwith a wide range of input impedances. For example, voltage differencingdevices typically present a high impedance whereas current differencingdevices typically present a low impedance. This range of inputimpedances for the active differencing devices 400, 420 permits theactive differencing device 400, 420 to be used with the matchedimpedance connection 300 as described in connection with FIG. 3A, thehigh impedance receive path signal connector 320 described in connectionwith FIG. 3B, or the high impedance transmit signal connector 340described in connection with FIG. 3C.

FIG. 4C illustrates a passive electronic differencing device 440.Passive devices are limited to having a gain less than one, and thus allhave some loss. Consequently, passive differencing devices 440 havehigher noise figures than the active electronic differencing devices400, 420 described in connection with FIGS. 4A and 4B. There are manyways to implement a passive electronic differencing device. For example,lumped element resistive dividers, traveling wave resistive (Wilkinson)dividers, and 180 degree hybrid couplers are all effective atimplementing an electronic differencing device.

Active electronic differencing devices, such as the devices 400, 420described in connection with FIGS. 4A and 4B, can be used to sum twosignals. Differencing can be realized by offsetting the phase of theclean transmit signal by 180 degrees relative to the phase of thetransmit signal that is applied to the antenna 204, which effectivelyapplies the inverse of the transmit signal to the summing port. Thisequivalence between subtracting and adding the inverse is easilydemonstrated by the equality: Rx−Tx=Rx+(−Tx). In some embodiments of thepresent teaching, the same physical hardware can realize both thematched signal connector described in connection with FIG. 3A 300 andthe passive differencing device 440 which implements 180 degree phasereversal of the clean transmit signal where necessary as described inconnection with FIG. 4C.

FIG. 4D illustrates one embodiment of a photonic differencing device 460that includes a balanced-drive optical modulator which produces amodulated output that is proportional to the sum or difference betweenthe signals that are applied to the electrodes. Such electrodes can beeither high impedance or matched impedance so that the photonicdifferencing device can be used with the matched impedance signalconnector 300 described in connection with FIG. 3A, the high impedancereceive path signal connector 320 described in connection with FIG. 3Bor the high impedance transmit signal connector 340 described inconnection with FIG. 3C.

Furthermore, depending on the design of the particular photonicdifferencing device the photonic differencing device can have a gainthat is greater or less than unity. Thus, the photonic differencingdevice can provide either gain or loss. When the photonic differencingdevice is designed to have gain, it is capable of achieving low noisefigure, much like active electronic differencing devices. When thephotonic differencing device is designed to have loss, it has highernoise figure, much like passive electronic differencing devices. Sometypes of differential optical modulators are only capable of summing twosignals. In such cases, these differential modulators can realize therequired differencing by offsetting the clean transmit signal by 180degrees as described in connection with FIG. 4C.

There are two basic types of signal sources: voltage sources and currentsources. An ideal voltage source is a signal source with zero internalimpedance. An ideal current source is a signal source with infiniteinternal impedance. Such ideal signal sources are not realizable.Realizable voltage sources generally have an internal impedance that ismuch lower than the external impedances in the circuit. Realizablecurrent sources generally have an internal impedance that is much largerthan the external impedances in the circuit.

FIG. 5 illustrates various signal isolators that can be used with thesame-aperture any-frequency STAR system of the present teaching. FIG. 5Aillustrates an electronic voltage-source-based isolator 500. Theisolator 500 in FIG. 5A shows one simple way that isolation can beachieved with a voltage source. A voltage source establishes a potentialdifference or voltage across its output terminals. The voltage across avoltage source is independent of an external signal that is applied toits output. Hence the current that is developed through a resistorconnected in series with a voltage source will not change the outputvoltage of the voltage source. For same-aperture any-frequency STARsystems, the voltage source signal is the transmit signal and theexternally applied signal would be the receive signal. Consequently, theoutput of the voltage source will contain a clean copy of the transmitsignal, which is what is desired.

FIG. 5B illustrates a current-source-based signal isolator 520 that canbe used with the same-aperture any-frequency STAR system of the presentteaching. Current sources establish a current that is independent of anexternal signal applied to its output. Hence, the voltage that developsacross a resistor that is connected in series with a current source willonly contain the current source signal and will not contain any signalthat corresponds to the externally applied signal. For same-apertureany-frequency STAR systems, the current source signal is the transmitsignal and the externally applied signal is the receive signal.Consequently, the voltage across the resistor will contain a clean copyof the transmit signal, which is what is desired.

FIG. 5C illustrates a non-reciprocal RF isolator 540 that can be usedwith the apparatus of the present teaching. Examples of non-reciprocalRF isolators are ferrite isolators and gyrators. These devices have lowtransmission loss in one direction and high transmission loss in theother direction. For example, there can be low transmission loss fromport 1 to port 2, but high transmission loss in the other direction,from port 2 to port 1.

FIG. 5D illustrates a photonic isolator 560 that can be used with thesame-aperture any-frequency STAR system according to the presentteaching. Photonic isolators provide good coupling in the forwardcoupling direction and high isolation in the reverse direction. Goodcoupling in the forward direction is accomplished by anelectrical-to-optical conversion device, such as a diode laser or anoptical modulator, whose optical output is efficiently coupled to anoptical-to-electrical conversion device, such as a photodetector.Photonic isolators provide extremely low coupling in the reversedirection because devices such as photodetector do not emit light, andthe electrical-to-optical conversion device is not capable of detectinglight.

FIGS. 6A-6D illustrate signal processors 600, 620, 640, and 660 that canbe used with the same-aperture any-frequency STAR system according tothe present teaching. Various types of digital and/or analog signalprocessors 600, 620, 640, and 660 can be used as shown in FIGS. 6A-6D.Referring to FIGS. 2 and 6, the signal processors 600, 620, 640, and 660execute a wide range of algorithms, such as a least mean squarealgorithm, to perform various functions. The signal processing can beperformed on the radio-frequency (RF) transmit signal, the RF receivesignal, or at some lower intermediate-frequency (IF) signals. One suchfunction is to correlate the clean copy of the transmit signal with theoutput of the differencing device 210, which contains both receive andtransmit signals. The result of this correlation will be a residualtransmit signal that is present in the output of the differencing device210.

Another function performed by the signal processor 212 is estimating thecomplex value of the transmit signal that needs to be applied to theinput of the differencing device 210 so as to result in minimizing theresidual transmit signal at the output of the differencing device 210.The result of this estimation is a signal that is applied to thetransmit signal adjustment circuit 214.

FIG. 7 illustrates transmit signal adjustment circuits 700, 720 that canbe used with the same-aperture any-frequency STAR system according tothe present teaching. The transmit signal adjustment circuits 700, 720generate a signal that adjusts the complex value of the transmit signal.There are numerous types of signal adjustment circuits that can be usedwith the same-aperture any-frequency STAR system according to thepresent teaching, two of which are shown in FIGS. 7A and B. FIG. 7Aillustrates an embodiment of an adjustment circuit 700 that adjusts themagnitude and phase of the transmit signal. FIG. 7B illustrates anembodiment of an adjustment circuit 720 that adjusts the in-phasein-quadrature components of the transmit signal.

FIG. 8 illustrates a block diagram of a front-end system 800 thatincludes the matched impedance signal connector 802, the photonicdifferencing circuit 804 and the voltage-source isolator 806 asdescribed herein. The system 800 passively reduces and ultimately eveneliminates the need for the transmit signal adjustment device and thesignal processor described herein. To accomplish this goal, the circuitson the two sides of the differencing device 804 are made as identical aspossible. To this end, a pseudo-antenna 808 can be constructed, which isa circuit that replicates as closely as possible the impedance vs.frequency function of the antenna 810.

To further establish as good a balance as possible between the twoinputs to the differencing device, identical connectors are used, inthis case, the matched impedance type can be used. This example systemuses the photonic differencing device described herein. Key advantagesof this type of differencing device or subtractor are that they areextremely wide bandwidth (>4 decades) and there is high isolationbetween the + and − differencing ports. Voltage source isolation, withidentical output impedance in the two outputs, further enhances thebalance. One of the disadvantages of this system architecture is therelatively high loss incurred by the transmit signal. Because the sametransmit power is supplied to both the antenna and the pseudo-antenna,there is 3 dB of loss for ideal (i.e., lossless) connectors. There is anadditional 3 dB loss at each of the connectors. Thus, the total transmitloss between the output of the power amplifier and the antenna is 6 dBplus the excess loss of the connector.

FIG. 9 illustrates a block diagram of one exemplary embodiment of afront-end system 900 that includes the signal connector 902 to which ahigh impedance is presented by the output of the transmit signal path,the passive electronic differencing device 904, and thecurrent-source-based isolator 906 described herein. This is compatiblewith the version of the connector that has a high impedance on the portthat connects to the transmit path output. In this system 900, theimpedances on the other two connector ports are matched: the antennaport provides the load to the antenna 908 and the differencing port isloaded by one input to the differencing device 904, which in this system900 is of the passive electronic type. The passive electronicdifferencing device 904 has a narrower bandwidth than the photonicdifferencing device described herein. However, it has slightly lowertransmit loss: 4.77 dB ideally, vs. 6 dB for the architecture shown inFIG. 8.

FIG. 10 illustrates a block diagram of one exemplary embodiment of afront-end system 1000 that includes a signal connector 1002 with a highimpedance applied to the output receive signal port by the ‘+’ port ofthe active electronic differencing device 1004, and thevoltage-source-based isolator 1006 described herein. One potentialadvantage of differencing devices 1004 of this type is that the inputimpedance can be made higher than the system impedance. For example, acommon system impedance is 50Ω. The input impedance of the activeelectronic differencing device 1004 can range from 500Ω for someimplementations to >1 MΩ for other implementations. This means that thesignal power drawn by the differencing device inputs can be negligible.Therefore, it is advantageous to select a connector type that isdesigned to work with a high impedance at its port that feeds thedifferencing device and to use the voltage type of isolators whoseisolated output is designed to feed a high impedance. Hence the systemconfiguration that is shown in FIG. 10 contains both a voltage sourceisolator 1006 and a signal connector 1002 with high impedance at itsdifferencing device port. One of the key features of this implementationis that the transmit loss is now 0 dB, at least in the ideal case.

FIG. 11 shows a same-aperture any-frequency STAR system 1100 using afast switch as a signal connector. The configuration leads to aparticularly simple implementation of the present teaching. The basis ofoperation of this implementation can be understood as follows. It iswell known to those in the art that a continuous signal can becompletely characterized by sampling it at a rate that is at least twiceits maximum frequency component. This is often referred to as theNyquist sampling theorem. One of the consequences of Nyquist sampling isthat it is not necessary to continuously monitor a continuous signal:observing—i.e., sampling—a continuous signal at its Nyquist rate issufficient. Instantaneous sampling—i.e. in zero time—is obviously atheoretical abstraction. For practical engineering purposes, a sample isconsidered to be instantaneous if the length of the sampling interval isshort compared to the interval between samples. For example, a samplingpulse that lasts for even 1% of the time interval between samples isoften considered as sufficiently short that it approximates thetheoretically ideal sampling.

To implement sampling, one can use a fast switch that is capable ofconnecting the input—in this case the signal coming from the antenna1102—to the receiver for the short period of time of the sample, andthen opening—i.e. disconnecting the input from the receiver. This meansthat for the remaining 99% of the time between samples, the samplingswitch is open, and hence the receiver is not connected to the input.The fast switch connector 1100 utilizes the inter-sampling interval toconnect the transmitter to the antenna 1102. There is negligibletransmitter power loss since the transmitter is connected to the antenna1102 for almost 100% of the time. With the fast switch signal connector,the transmitter and the receiver are never simultaneously connected tothe antenna 1102. Hence, the transmit signal does not have theopportunity to enter the receive path. This can eliminate the need forthe differencing device, isolator, signal processor and transmit signaladjuster described herein for some applications

It is important to point out that, while the fast switch istopologically similar to a conventional transmit-receive (T/R) switch insystems not designed for STAR, the function of the fast switch connectorshown in FIG. 3D is distinct. In the case of a conventional T/R switch,the switch only needs to operate with speeds between tens ofmilliseconds and one second. Hence a conventional T/R switch does notoperate fast enough to perform the sampling function, which is centralto the present operation.

Although in some system applications, sufficient performance may beachievable using the same-aperture any-frequency STAR systems describedin FIGS. 2-11, in other system applications it will be necessary toaugment the front end performance with signal processing techniques. Inany embodiment of the present teaching, signal processing can beincorporated with the front end to achieve enhanced performance. As willbe evident to those skilled in the art, it is possible to augment any ofthe front end systems described herein with signal processing; weillustrate this by selecting to augment the example front end systemarchitecture shown in FIG. 10.

FIG. 12 shows a block diagram of a same-aperture any-frequency STARsystem 1200 using digital signal processing 1202 to augment the examplefront end system shown in FIG. 10. This example system uses the digitalsignal processor with down conversion described in connection with FIG.6B. Also, this example system uses the vector modulator type of transmitsignal adjuster described in connection with FIG. 7B. A portion of theoutput of the differencing device is fed to a downconverter 1206 thattranslates the frequency spectrum of the signal down to a lowerfrequency, which can be an intermediate frequency (IF). Alternatively,it can be translated all the way down to zero frequency, which is morecommonly referred to as baseband. A portion of the transmit signal isalso downconverted, with the constraint that it be converted to the samefrequency to which the output of the differencing device was converted.Once both these signals have been downconverted, they are converted todigital form via analog-to-digital converters (ADC) 1204.

In the digital domain, the digital signal processor 1202 is used tocorrelate the transmit signal with the differencing device 1208 outputto isolate the residual transmitter component in the differencing device1208 output. The signal processor 1202 then forms an estimate of theoptimum complex value of the transmitter signal that needs to beinjected into the differencing device 1208 so as to minimize theresidual transmitter signal that is present at the differencing device1208 output. The output of the signal processor 1202 includes twosignals that contain the desired settings on the transmit signaladjuster. Since in this example, we are using a vector modulator, thecomplex settings are for the in-phase (I) and quadrature (Q) portions ofthe transmitter signal. Since many vector modulators require analoginputs, FIG. 12 shows digital-to-analog converters (DACs) 1210 toexecute the required conversion.

FIG. 13 shows a block diagram of a same-aperture any-frequency STARsystem 1300 illustrating how analog signal processing could be used toaugment the example front end system shown in FIG. 10. For this examplesystem, we have selected the analog signal processor 1302 withoutfrequency conversion as described in connection with FIG. 6C. Since thetransmitter and differencing device outputs are analog signals, theanalog signal processor 1302 does not require analog-to-digitalconverters. One way that the required processing can be performed iswith an integrated circuit that contains many of the functions, such asthe AD8333, which is a dual I/Q demodulator commercially available fromAnalog Devices. The required analog multiplications also can beperformed with integrated circuits, such as the AD835, which is avoltage-output, 4-quadrant multiplier also commercially available fromAnalog Devices. The output of the analog multipliers, with appropriatesumming, scaling and integration, can drive the vector modulator inputsdirectly without the need for digital-to-analog coverers.

All of the embodiments of the present teaching in FIGS. 2-13 would beeffective at removing the high-power transmit signal from the receivepath. If the transmit signal strength is only of the same order ofmagnitude as, or smaller in magnitude than, the receive signal, thenmuch less hardware may be required.

FIG. 14 illustrates a subset system 1400 of hardware in thesame-aperture any-frequency STAR system described in connection withFIG. 2 that is useful for some embodiments when the transmit signalstrength is only as strong as or weaker than the receive signal. Athree-port signal connector 1402 is necessary to permit connection ofthe separate transmit path 1404 and receive signal path 1406 to theantenna 1408, and the isolator 1410 is necessary to shield the transmitpath 1404 from the signal environment in which the antenna operates. Ananalog signal differencing device, however, may not be required, andthus neither would the transmit signal adjuster be required. Because thetransmit signal is relatively small, it does not saturate any of thecomponents in the receive signal path, and its removal from the receivesignal path, if deemed necessary, can be accomplished using well-knowndigital signal processing techniques.

FIG. 15 illustrates one exemplary system 1500 which is an embodiment ofthe system 1400 in FIG. 14, including a signal connector 1502 to which ahigh impedance is presented by the output of the photonic isolator 1504in the transmit signal path 1506, and in which a conventional digitalsignal processor 1508 is used to remove the transmit signal from thereceive path 1510 after all signals are frequency down-converted andthen converted from the analog to digital domain. In the case ofnon-cooperative interfering sources, a reference copy of the interferersthat is fed to the interference canceller must be self-generated. Exceptfor the fact that problematic interferers are large in amplituderelative to the signal of interest (SOI), we often cannot assume we knowanything else about these interferers at all. Therefore, to generate areference copy of the interferers requires a way of sensing only thelarge interferers that may be present and ignoring the SOI. A knownmethod to preferentially detect the interfering source is to usedirectional antennas whose maximum sensitivity is pointed in thedirection of the interfering source. The effectiveness of suchtechniques, however, is heavily dependent on the directionality of theantenna beam and the angle separation between the interfering source andthe SOI. Therefore, one feature of the present teaching is an approachfor extracting a reference copy of a strong interfering signal from thecomposite SOI+interfering signal stream that is coming from an antenna.

FIG. 16 illustrates a system 1600 that generates a reference copy of aninterfering signal according to the present teaching. A portion of theantenna 1602 output is tapped off and fed to an N-bit quantizer 1604,where N is sufficient to quantize the strong interferer but notsufficiently large to also quantize the SOI, which is much smaller thanthe interfering signal. In this way, the N-bit quanitzer serves as asort of a reverse limiter, letting only large signals through andsuppressing smaller signals. The delay involved in producing a referencecopy of the interferers in this way and processing it in theinterference canceller can be reproduced in the signal path leading fromthe antenna 1602 to the interference canceller as shown in FIG. 16. Todemonstrate the operation of the self-generated reference, simulationswere performed, in which the “high-power” interferer was a 1-V sine waveat 100 MHz and the “low power” SOI was a 0.1-V sine wave at 107 MHz.

FIG. 17 illustrates results of a simulation of the architecture in FIG.16, whereby the output of a 1-bit quantizer produces a copy of thehigh-power interferer at 100 MHz, allowing its subtraction from thelower-power 107-MHz SOI in a differencing device. The plots illustratedin FIG. 17 show the input to the N-bit quantizer, while the main plotshows the output of the quantizer, with the number of bits as aparameter. With the interferer only a factor of 10 times stronger thanthe SOI, a single bit of quantization “passes” the large interferer andcompletely fails to sense the smaller SOI, and 4 bits are sufficient tocompletely sense both the interferer and SOI. With 2 bits, the SOI is˜20 dB below the high-power interferer.

Given that we will wish to cancel large interferers with morecomplicated spectral content than the simple sinusoid, we assumed inthis simulation that we will need multiple bits of quantization topreserve this content. Thus, we will only be able to effectively cancelthe effect of interferers much (not just 10 dB) stronger than the SOI.

FIG. 18 is a plot that shows the relationship between thesignal-to-interferer ratio at the antenna to the number of bits ofquantization that we can use without having to worry about suppressingthe SOI, which is like throwing the SOI “baby” out with theinterferer(s) “bathwater.” The number of bits is a metric of thecomplexity of the interference signal spectrum.

FIG. 19 illustrates a block diagram of a system 1900 according to thepresent teaching that uses a self-generated reference in an interferencecanceller. The system 1900 includes the N-bit quantizer 1902 thatgenerates the reference copy of the interferer as described inconnection with FIG. 16. An analog processor 1904 and a digitalprocessor 1906 are used in a feedback loop with the large signaldifferencing device 1908 to remove the interfering signal. An RF delay1910 is used to match the delay between the + and − ports of thedifferencing device 1908.

FIGS. 2-19 relate to systems in which the transmitting and receivingantenna consists of one radiating element addressed by one front-end.Alternatively, the antenna symbol in FIGS. 1, 2, 8-16, and 19 canrepresent an array of radiating elements all being fed the same transmitsignal by a single front-end and having their received signals combinedfor processing in that same front-end. In many practical systems, it ismore advantageous, however, for each radiating element or small group ofradiating elements in a large array of such elements to be addressed byits own front-end. In this case, each front-end may need to mitigate theeffect of the presence in its receive signal path of not only thetransmit signal being transmitted by its radiating element or smallgroup of radiating elements, but also by the signals being transmittedby any or all of the other elements in the array whose transmittedsignals will be received in part by this front-end's antenna elementthrough a phenomenon known in the art as mutual coupling between antennaelements.

FIG. 20 illustrates a system 2000 according to the present teaching formitigating the effect of signals being transmitted not only by theantenna element 2002 attached to the front-end 2004 shown by thecollection of hardware inside dashed box but also by the N−1 otherradiating elements in an array of N such radiating elements.

The system 2000 is a generalized form of the single-element front-enddescribed in connection with FIG. 2. The difference between the twofigures is noticeable in that there are now a number N rather than onlyone transmit signal adjuster. That is one for each of the N elements inthe antenna array. The copy of the transmit signal which, in thesingle-antenna-system front-end of FIG. 2, the isolator 2004 provides toa single transmit signal adjuster 2006, is now split into N parts by anN-way RF divider 2008, such as two-way traveling-wave resistive powerdividers (Wilkinson dividers) employed in a corporate tree arrangementto yield N-way splitting of the signal.

One of the N attenuated (by at least a factor of N) copies of thetransmit signal is fed to this front-end's transmit signal adjuster 2006exactly as was done in FIG. 2. The remaining N−1 attenuated copies ofthis front-end's transmit signal are routed out of this front-end, andeach is connected to one of the transmit signal adjusters 2006 in eachof the N−1 other antenna element 2002 front-ends 2004. Correspondingly,each of the other N−1 transmit signal adjusters 2006 in the oneelement's front-end 2004 shown in FIG. 20 receives an attenuated copy ofthe signal being transmitted by the other N−1 antenna elements 2002.These signal adjuster's 2006 outputs are combined, along with the outputof the transmit signal adjuster 2006 that acts upon this antennaelement's transmit signal, in an N-way RF combiner 2010, as shown inFIG. 20.

Identical to the N-way RF divider 2008, this N-way RF combiner 2010 mayconsist, for example, of two-way traveling-wave resistive powercombiners (Wilikinson power combiners) employed in a corporate treearrangement to yield N-way combining of the RF signals. The combinedcopies of the transmit signals are subtracted from the signal receivedby this front-end's antenna element 2002 in the differencing device2012. As in FIG. 2, the output of the differencing device 2012 is fed toa signal processor 2014. Additionally, the signal processor 2014receives its own attenuated copy of each element's transmit signal asthe signal processor in FIG. 2 does. For clarity, however, this featureis not shown in FIG. 20, but will be understood by those skilled in theart.

What is claimed is:
 1. An any-frequency simultaneous transmit andreceive (STAR) system, the system comprising: a) a first antenna elementthat transmits a first RE transmit signal and that receives an RFreceive signal; b) a second antenna element that transmits a second RFtransmit signal, wherein the first and second RF transmit signals andthe RF receive signal occupy any RF frequency; c) a signal connectorconfigured to connect a path to and from the first antenna element, apath from an output of a transmit path and an input of a receive path;d) a first transmit signal adjuster comprising an input connected to thetransmit path; e) a second transmit signal adjuster, wherein an outputof the first transmit signal adjuster and an output of the secondtransmit signal adjuster are combined at an output of a combiner; f) asignal differencing device comprising a first input connected to thereceive path and a second input connected to the output of the combiner,the signal differencing device being configured to receive combinedcopies of the first and second RF transmit signals and to subtract themfrom the RE receive signal, thereby mitigating an effect of both thefirst and the second RF transmit signals on the RF receive signal; andg) a signal processor comprising an input connected to an output of thesignal differencing device, an output connected to an input of the firsttransmit signal adjuster and connected to an input of the secondtransmit signal adjuster and connected to a receive output, the signalprocessor being configured to receive attenuated combined copies of thefirst and second RF transmit signals and to determine a complex value ofat least one of the first and second RF transmit signals so as tominimize at least one of residual first and second RF transmit signalsin the RF receive signal at the receive output.
 2. The system of claim 1wherein the signal connector provides a matched impedance for at leastone of the path from and to the first antenna element, the path from theoutput of the transmit path, and the path to the input of the receivepath.
 3. The system of claim 1 wherein the signal connector provides ahigher impedance at the path from the output of the transmit path thanat either of the path to and from the first antenna element and the pathfrom the output of the transmit path.
 4. The system of claim 1 whereinthe signal connector comprises a fast switch.
 5. The system of claim 1wherein signal differencing device comprises an active differencingdevice.
 6. The system of claim 1 wherein the signal differencing devicecomprises a passive differencing device.
 7. The system of claim 1wherein the signal differencing device comprises a photonic differencingdevice.
 8. The system of claim 1 wherein an algorithm executed by thesignal processor correlates a copy of at least one of the first andsecond RF transmit signal with the output of the differencing device andgenerates a more accurate representation of the RF receive signal. 9.The system of claim 1 wherein the signal processor comprises an analogsignal processor.
 10. The system of claim 1 wherein the signal processorcomprises a digital signal processor.
 11. The system of claim 1 whereinthe first and second RF transmit signal comprise a same transmit signal.12. The system of claim 1 wherein the first and second RF transmitsignal comprise a different transmit signal.
 13. The system of claim 1wherein the system comprises a microwave system.